SAV Controller described pilot deviation event when RADAR target was not observed while over flying the RADAR site; the aircraft descending without clearance; causing a near separation loss.

Narrative

While I was working North Radar; West Radar; and Associate Handoff position; I took handoff on Aircraft X at 7;000 FT and asked him 'I will need you at 6;000 FT or 8;000 FT in a few miles; which would you like?' He said he would like 6;000 FT. I told him to expect that in the next few miles. I noticed he went over the radar site and was not on the scope. While coordinating a point out without approval or my knowledge; the pilot descended on his own and popped back up on the radar scope at 6;000 FT. When I noticed it; I took immediate action to rectify the situation via climbing and turning (R1) and issue traffic. My worse nightmare with the radar has happened to me. The problems with the radar have now caused a safety issue. Even though the past 3 years airways facilities has been against getting an ASR-11; they have changed their tone. During the meeting the Airways Facilities Manager stated that the ASR-8 was horrible and needs to be replaced and also said that all the local congressmen know that it is not reliable.